Detected by Coverity.  The fix in 2c27dfae didn't catch all bad
instances of memcpy().  Thankfully, on further analysis, all of
the problematic uses are only triggered by old qemu that lacks
-device.

* src/qemu/qemu_hotplug.c (qemuDomainAttachPciDiskDevice)
(qemuDomainAttachNetDevice, qemuDomainAttachHostPciDevice): Init
all fields since monitor only populates some of them.